x86/kvm: Pass stable clocksource to guests when running nested on Hyper-V

Currently, KVM is able to work in 'masterclock' mode passing
PVCLOCK_TSC_STABLE_BIT to guests when the clocksource which is used on the
host is TSC.

When running nested on Hyper-V the guest normally uses a different one: TSC
page which is resistant to TSC frequency changes on events like L1
migration. Add support for it in KVM.

The only non-trivial change is in vgettsc(): when updating the gtod copy
both the clock readout and tsc value have to be updated now.

Signed-off-by: Vitaly Kuznetsov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: [email protected]
Cc: Radim Krčmář <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: "Michael Kelley (EOSG)" <[email protected]>
Cc: Roman Kagan <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: [email protected]
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Cathy Avery <[email protected]>
Cc: Mohammed Gamal <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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